Previously, planar transmission lines such as conventional microstrip, stripline, and co-planar waveguide (CPW) have been utilized to construct conventional structures when designing microwave and millimeter wave circuits. These transmission lines allow for the fabrication of passive components having predefined electrical functions and further enhance the ease in mounting active devices through their conventional structures. Almost forty years ago the stripline was introduced as a new and revolutionary hybrid technology which has evolved and is presently applied to monolithic technologies where it drastically increases operating frequencies and also consequently reduces weight and volume. In conventional planar transmission lines such as the co-planar waveguide, power is propagated by creating an RF voltage difference between a pair of planar conductors printed on a common surface. Alternatively, both stripline and microstrip propagate power by creating an RF voltage difference between two planar conductors printed on opposite surfaces of a dielectric slab structure. In both of these cases, the geometry of the conventional planar transmission lines permits greater design flexibility, tremendously reduces space occupied by the circuit, and contributes to realizing very large scale, very high frequency applications. Although microstrip and stripline have been utilized more so in passive circuits, limitations in mounting active devices have made the use of co-planar waveguides more popular since its physical geometry accommodates the incorporation of active devices. However, it is commonly observed that a degradation in circuit performance results from circuit crowding with active and passive devices due to coupling mechanisms associated with parasitics which are excited along with radiation effects that arise when utilizing the above in dense circuit environments.
Whereas the planarization of conductors by the aforementioned transmission lines provides integration capabilities, fringing is generated in the electro-magnetic fields which leads to unwanted mechanisms such as radiation and dispersion, and enhanced OHMIC losses and electromagnetic coupling. Each of these mechanisms are dependent on the frequency, and become seriously limited as the submillimeter frequency range is approached. An effort was made to find new geometries which reduce or eliminate the aforementioned losses and coupling mechanisms but do not affect the monolithic character of the transmission line, therefore allowing for extension of operating frequencies long into the Terahertz region, thereby improving circuit performance in existing applications. Typically, planar circuits have been enclosed in shielding cavities in order to resolve these problems. However, in most cases, the cavities must be placed away enough from the circuit in order to avoid proximity effects and they must be sufficiently small enough to avoid cavity-resonances that interfere with circuit electrical performance.
Furthermore, most high frequency circuits are presently developed before they are packaged in a shielding housing wherein the development, modeling, fabrication and experimental characterization of the systems are performed prior to packaging. Therefore, the effects of a housing on the electrical performance of the developed circuit becomes very difficult to predict. As a result, the electrical response of many packaged circuits suffers significant performance degradation mainly due to the introduction of unwanted parasitics along with the excitation of multiple shielding resonances resulting from the interaction between a circuit board and a metallic housing. Furthermore, while shielding with metallic housings is possible in many circuit applications, in monolithic arrays where the circuit environment must remain open, radiation from feeding structures and parasitic coupling to radiating elements has been a major problem.
A previous effort by one of the present inventors involved the development of a microshield line in an attempt to improve performance over dimensional microstrip or co-planar structures in order to reduce radiation losses and electromagnetic interference. The microshield line is a monolithic line suitable for circuit or array applications. With this device, an inner conductor is coupled with a ground plane which is deformed from the original planar geometries of previous devices in order to totally or partially surround the inner conductor while still having the form of a printed strip. The structure is generally made monolithically using etching and metal deposition techniques. In the preferred version, the inner conductor is suspended in air by mounting the conductor on a membrane. Typically, the ground plane surrounds the inner conductor and prevents radiation effects. However, conductor loss is still present with this structure. Furthermore, circuit implementations with this device are limited to cases where a circuit can be suspended in air by use of a membrane. While such a construction facilitates hybrid use with some passive and active circuit components, it does not allow for the construction of more complex circuits due to limitations on the size of the fabricated membrane, as well as the limited structural support provided by the membrane to the circuit elements.